机械工程导论双语课件.pptx

单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,#,单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,#,单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,#,单击此处编辑母版标题样式,mechanical,engineering,An,introduction to,1,Lecturer:Liu Ju-rong,CHAPTER2 Problem-Solving Skills,OVERVIEW,1,UNIT,SYSTEMS AND CONVERSIONS,2,3,DIMENSIONAL CONSISTENCY,2,SIGNIFICANT DIGITS,4,AN ERROR OF UNITS ON THE WAY TO MAR,5,APPROXIMATION IN ENGINEERING,6,PROBLEM-SOLVING METHODOLOGY,7,Vocabulary,3,Torque,扭矩,thermal conductivity,热导率,shear stress,剪应力,fluid viscosity,流体的粘度,elastic modulus,弹性模量,kinetic energy,动能,Reynolds number,雷诺数,1 OVERVIEW-the important of unit,4,How important a unit is?,Each quantity in mechanical engineering has two components:a numerical value and a unit.,One is simply meaningless without the other,and practicing engineers pay as close and careful attention to the units in a calculation as they do to the numbers.,1 OVERVIEW-,The ability to be,available in this chapter,5,After study,what,should,you be,able,to have?,Report both a numerical value and its unit in each calculation that you perform.,List the base units in the United States Customary System and the System International d Units,and state some of the derived units used in mechanical engineering.,Understand the need for proper bookkeeping of units when making engineering calculations and the implications of not doing so.,Convert,numerical quantities from the United States Customary System to the System International d Units,and vice versa.,Check your equations and calculations to verify that they are dimensionally consistent.,Understand how to perform order-of-magnitude approximations.,2,UNIT SYSTEMS AND,CONVERSIONS-Unit System,what is USCS and SI?,6,Engineers specify physical quantities in two different,but conventional,systems of units:the United States Customary System(USCS)and the International System of Units.,Practicing mechanical engineers must be conversant with both unit systems.,2,UNIT SYSTEMS AND,CONVERSIONS-,an example,Keeping Track of Units on Fright,143,7,In July of 1983,Air Canada Flight 143 was en,route from,Montreal to Edmonton.The Boeing 767 had three fuel tanks,one in each wing and one in the fuselage,which supplied the planes two jet engines.,volume(L),weight(lb),mass(kg),Total fuel,22,300,kilograms(kg),How much fuel should be added,?,Fuel in tanks,7682,liters(L),Boeing 767,pounds(lb);,kilograms,1.77 lb/L,1.77 kg/L,9000,liters,16,000,liters,2,UNIT SYSTEMS AND,CONVERSIONS-Base and Derived Units,what is base units?,What is derived units?,8,A base unit is a fundamental quantity that cannot be further broken down or,expressed,in terms of any simpler elements.Base units are the core building,blocks of,any unit system.,Derived units,as their name implies,are constructed as combinations of base units.,9,2 WHAT IS ENGINEERING,?-,United States Customary System,The United States Customary System of units is a historical and traditional system,and its origin traces back to the ancient Roman Empire,.,Why does the United States stand out in retaining the USCS?,The reasons are both logistical and cultural:There is already a vast continent-sized infrastructure within the United States that is based on the USCS.,Conversion away from the existing system would be a significant and expensive burden.,The dimensions of countless existing structures,factories,machines,and spare parts have already been specified and built in terms of the USCS.,10,2 WHAT IS ENGINEERING,?-,United States Customary System,Quantity,USCS Base Unit,Abbreviation,Length,foot,ft,Force,pound,lb,Time,second,s,Electric current,ampere,A,Thermodynamic temperature,Rankine,R,Amount of substance,mole,mol,Luminous intensity,candela,cd,TABLE 2.1 Base Units in the USCS,11,2 WHAT IS ENGINEERING,?-,United States Customary System,TABLE 2.2 Certain Derived in the USCS.Although a change in temperature of 1 Rankine also equals a change of 1 degree Fahrenheit,the absolute values are converted using the formula.,Quantity,USCS Derived Unit,Abbreviation,Definition,Length,mil,mil,1 mil=0.001 in.,Length,inch,in.,1 in.=0.0833ft,Length,yard,yd,1 yd=3 ft,Length,mile,mi,1 mi=5280 ft,Volume,U.S.gallon,gal,1 gal=0.1337 ft,3,Mass,slug,slug,1 slug=1 lb,s,2,/ft,Force,ounce,oz,1 oz=0.0625 lb,Force,ton,ton,1 ton=2000lb,Moment,of a force,ft,lb,ft,lb,-,Pressure or stress,psi,psi,1 psi=,1 lb/in,2,Energy,work,or heat,ft,lb,ft,lb,-,Energy,work,or heat,Btu,Btu,1 Btu=778.2 ft,lb,Power,horsepower,hp,1 hp=550 ft,lb/s,Temperature,degree Fahrenheit,F,F=,R-460,12,2 WHAT IS ENGINEERING?-International System of Units,The International System of units(or SI)is the measurement standard based in part on the quantities of meters,kilograms,and seconds.,Quantity,SI Base Unit,Abbreviation,Length,meter,m,Force,kilogram,kg,Time,second,s,Electric current,ampere,A,Thermodynamic temperature,Kelvin,K,Amount of substance,mole,mol,Luminous intensity,candela,cd,TABLE 2.3 Base Units in the SI,13,2 WHAT IS ENGINEERING?-International System of Units,Quantity,SI Derived Unit,Abbreviation,definition,Length,micron,m,1,m=10,-6,m,Volume,liter,L,1 L=0.001 m,3,Force,newton,N,1 N=1kg,m/s,2,Moment of a force,N,m,N,m,-,Pressure or,stress,pascal,Pa,1 Pa=1 N/m,2,Energy,work,or,heat,joule,J,1 J=1 N,m,Power,watt,W,1 W=1 J,temperature,degree Celsius,=,K-273,TABLE 2.4,Certain Derived in the,SI.,Although a change in temperature of 1,Kelvin,also equals a change of 1 degree,Celsius,the absolute values are converted using the formula,.,14,2 WHAT IS ENGINEERING?-International System of Units,TABLE 2.5 onversion Factor Between Certain Quantities in the USCS and SI,Quantity,To Convert from,To,Multipy by,Length,foot(ft),meter(m),0.3048,Length,inch(in.),meter(m),0.0254,Length,mile(mi),kilometer(km),1.609,Volume,gallon(gal),meter,3,(m,3,),3.78510,-3,Volume,gallon(gal),liter(L),3.875,Mass,slug,kilogram(kg),14.59,Force,pound(lb),newton(N),4.448,Pressure or stress,pound/inch,2,(psi),pascal(Pa),6895,Energy,work,or heat,foot-pound(ft,lb,),joule(J),1.356,Power,foot-pound/second,(ft,lb/s,),watt(W),1.356,Power,horsepower(hp,),kilowatt(kW),0.7456,15,2 WHAT IS ENGINEERING?-International System of Units,TABLE 2.6 onversion Factor Between Certain Quantities in the SI and USCS,Quantity,To Convert from,To,Multipy by,Length,meter(m),foot(ft),0.3048,Length,meter(m),inch(in.),0.0254,Length,kilometer(km),mile(mi),1.609,Volume,meter,3,(m,3,),gallon(gal),3.78510,-3,Volume,liter(L),gallon(gal),3.875,Mass,kilogram(kg),slug,14.59,Force,newton(N),pound(lb),4.448,Pressure or stress,pascal(Pa),pound/inch,2,(psi),6895,Energy,work,or heat,joule(J),foot-pound(ft,lb,),1.356,Power,watt(W),foot-pound/second,(ft,lb/s,),1.356,Power,kilowatt(kW),horsepower(hp,),0.7456,16,2 WHAT IS ENGINEERING?-International System of Units,TABLE 2.6 onversion Factor Between Certain Quantities in the SI and USCS,Name,symbol,Multiplicative Factor,tera,T,1,000,000,000,000,=10,12,giga,G,1,000,000,000,=10,9,mega,M,1,000,000,=10,6,kilo,k,1000,=10,3,hecto,h,100,=10,2,deca,da,10,=10,1,deci,d,0.1,=10,-1,centi,c,0.01,=10,-2,milli,m,0.001,=10,-3,micro,0.000,001,=10,-6,nano,n,0.000,000,001,=10,-9,pico,p,0.000,000,000,001,=10,-12,3,DIMENSIONAL,CONSISTENCY,17,Dimensional consistency means that the units associated with the numerical values on each side of the equality sign match.,In paper-and-pencil calculations,it is good practice to keep the units adjacent to each numerical quantity in an equation.,performing a double-check on the units in any equation is always a good idea.,4,SIGNIFICANT,DIGITS,18,A significant digit is a numerical value that is known to be correct and reliable in the light of inaccuracy that is present in the supplied information,any approximations that have been made along the way,and the mechanics of the calculation itself.,The precision of a number is half as large as the last significant digit used in expressing the number.,The factor of one-half,(,arises,because me last digit of a number represents the rounding-off,process either,higher or lower,of the trailing digits.,43.01 mN,43.01,mN,43.02,mN,5 AN,ERROR OF UNITS ON THE WAY TO,MARS,19,The importance of keeping track of units in engineering calculations was highlighted by the failure of the,Mars,Climate Orbiter spacecraft in 1999.,The spacecraft was to arrive at Mars on September 23,1999,and it was scheduled to complete its primary science mission on December 31,.,NASA conducted a thorough investigation,and the Mars Climate Orbiter Mishap Investigation Board identified eight factors that contributed to the spacecrafts loss.,units of(force),(time,),newton-seconds,pound-seconds,6 APPROXIMATION IN,ENGINEERING,20,Engineers are comfortable making reasonable approximations so that the systems they analyze are as simple as possible and yet will yield a result that is accurate enough for the task at hand.,Engineers nearly always make approximations when they design and solve technical problems.,Approximations are also useful to remove extraneous,features,that complicate the problem but otherwise have little influence on the final answer.,6 APPROXIMATION IN,ENGINEERING,21,Given the uncertainty present in real systems,it is often necessary for engineers to make order-of-magnitude approximations.,These are sometimes called back-of-the-envelope calculations because they can be performed quickly and informally.,7 PROBLEM-SOLVING METHODOLOGY,22,1.,Make,a clean start,2.Draw,3,.Givens and unknowns.,4.Think first,then write,.,5.Be coordinated,23,8.Significant figures,9.Box it,10.Interpret your answer,6.Neatness counts,.,7.Units.,7 PROBLEM-SOLVING METHODOLOGY,SUMMARY,24,Engineers are often described as being can-do people with excellent problem-solving skills.,In this chapter we have discussed some of the fundamental tools and skills that mechanical engineers use when they answer technical questions.,Numerical values,the USCS and SI systems,unit conversions,dimensional consistency,significant digits,and order-of-magnitude approximations are everyday issues for engineers.,Each quantity in mechanical engineering has two components-a numerical value and a unit-and it is meaningless to report one without the other.,Mechanical engineers need to be clear about those numerical values and units when they perform calculations,document results,and communicate their findings to others through written reports and oral presentations.,By following the consistent problem-solving guidelines developed in this chapter,you will be able to approach engineering problems in a systematic manner and be confident of the accuracy of your solutions.,SELF-STUDY AND REVIEW,25,What should you review?,What are the base units in the USCS and SI?,What are the examples of derived units in the USCS and SI?,How are mass and force treated in the USCS and SI?,How should you decide the number of significant digits to retain in a calculation an report in your final answer?,Summarize the major steps that should be followed when solving problems to clearly document your work and to catch otherwise avoidable mistakes.,26,Thank You!,